Like the other games, there are Pokémon that are just found in the overworld. Here, you just simply interact with them in order to capture them

#143 Snorlax

Location: Route 7

Method:
When you first enter Route 7, you will see that the road has been blocked. It seems a Snorlax has fallen asleep blocking the bridge. Knowing that Snorlax needs to be awake to be removed, you're tasked with getting the Poké Flute from the Parfum Palace. When you return with it, it is played and Snorlax attacks.